This invention relates to a disc driving apparatus for driving a disc-shaped recording medium, such as an optical disc, a magneto-optical disc or a magnetic disk, having plural servo patterns formed by wobble pits offset toward inner and outer circles of the recording track.
An optical disc of the sampled servo system as shown in the JP Patent Laid-Open (KOKAI) Publication No. 3-156774 has been conventionally known.
With an optical disc device of the sampled servo system, channel clocks, which are sampling clocks for data used in recording or reproduction, need to be phased correctly.
FIG. 1 shows the structure of a clock detection system for the optical disc as applied to a driving apparatus for a magneto-optical disc.
The magneto-optical disc employed in this driving apparatus has a number of servo areas provided at predetermined intervals, each servo area having servo patterns recorded therein. Each of the servo patterns is comprised of a pair of wobble pits 200 offset toward the inner and outer circles from the center of a concentric track TR and a clock pit 201 formed at the center of the track TR and in an intermediate position between the wobble pits 200, as shown in FIG. 2.
In each circle of the track, 960 servo areas are formed with an area defined between a given servo area and the next servo area serving as a data area. Recording data modulated in a predetermined manner are photomagnetically recorded in the data area along with synchronization data, address data and the like.
Referring to FIG. 1, the driving apparatus for the magneto-optical disc commences a servo pattern detection mode, in which the magneto-optical disc 101 having the above-described format is driven in rotation by a spindle motor 100 at a constant angular velocity as in recording of the data., so as to phase-match the servo pattern detection data with the channel clocks.
In the servo pattern detection mode, an optical pickup 102 causes the magneto-optical disc 101 to be irradiated with a laser beam, and then detects a return light beam therefrom. The servo patterns and the recording data recorded on the magneto-optical disc 101 are reproduced by photoelectric conversion of the return light beam, generating reproduction signals which are then supplied to an amplifier 105.
The amplifier 105 amplifies the reproduction signals with a predetermined gain to supply the amplified reproduction signals to an A/D converter 106. The A/D converter 106 is provided with channel clocks of free-running frequency from a voltage controlled oscillator (VCO) 111 which has an oscillation frequency changed in response to the voltage supplied thereto. The A/D converter 108 samples and digitizes the reproduction signals by the channel clocks to form reproduction data, which is then supplied to a latch circuit 107.
Each of the servo patterns is made up of a pair of wobble pits 200 and a clock pit 201 located in an intermediate position between the wobble pits 200 and at the center of the track TR, as shown in FIG. 2. Therefore, the reproduction data of the servo patterns have such a waveform that waveform portions of the wobble pits 200 lower in level than a waveform portion of the clock pit 201 are on both sides of the waveform portion of the clock pit 201, with the waveform portion of the clock pit 201 as the center, as shown in FIG. 3.
On the assumption that the channel clock from the VCO 111 has a correct phase, counting of the channel clocks is started after the servo pattern is detected. When the count values are equal to 277 to 279, 282 to 284 and 287 to 289, the servo patterns can be latched correctly by latching the reproduction data from the A/D converter 106.
For this reason, a servo pattern detection circuit, not shown, compares a pre-stored servo pattern with the reproduction data from the A/D converter 108, so as to detect the servo pattern and transmit a detection pulse thereof to a counter, not shown. The counter is reset by the detection pulse of the servo pattern, and at this point, starts counting the channel clocks. The count values are supplied to a latch pulse outputting circuit. The latch pulse outputting circuit transmits a latch pulse to the latch circuit 107 when the count values are equal to 277 to 279, 282 to 284 and 287 to 289.
The latch circuit 107 latches the reproduction data by each latch pulse to latch the reproduction data of the servo pattern, as shown in FIG. 3. The latch circuit 107 thus forms latch data at points a1, a0, a2, b1, b0, b2, c1, c0 and c2, and supplies these latch data to a phase generator 108.
The phase generator 108, thus provided with the reproduction data of the servo pattern, detects phase errors between the channel clocks and the servo pattern reproduction data, by the following Equation 1 based on level differences among points a1, a2, b1, b2, c1 and c2, which are shoulder points spaced apart by one channel clock ahead and behind the center points a0, b0 and c0, utilizing left-to-right symmetry of the three waveform portions. EQU phase error detection data=[(a2-a1)+(c2-c1)]/2 (1)
The phase generator 108 then supplies; the resulting phase error detection data to a D/A converter 109.
The servo pattern reproduction data are employed for formation of tracking error signals, formation of tracking polarity signals (TPOL), which rise to a high level when an error is within .+-.1/4 track from the track center, formation of one-eighth off-track signals, which rise to a high level when an error exceeds .+-.1/8 track from the track center, that is, when the value of a0 or c0 is greater than that of b0, formation of a mean level detection signal of the wobble pit, and formation of a level detection signal of a mirror area provided between the servo pattern and the data area, in accordance with the following Equations 2 to 6, respectively. EQU tracking error signal=c0-a0 (2) EQU 1/8 off-track signal=(b0&lt;a0)V(b0&lt;c0) (3) EQU TPOL=b0&gt;(a0+c0)/2 (4) EQU mean level detection signal of wobble pit=(a0+c0)/2 (5) EQU level detection signal of mirror area=d0 (6)
The D/A converter 109 converts the phase error detection data into analog signals to form phase error detection signals, which are then supplied to a phase compensation circuit 110.
The phase compensation circuit 110, which is formed by a low-pass filter or the like, removes high-frequency noise components of the phase error detection signal for phase compensation of the phase error detection signals, and then supplies to the resulting signals to the VCO 111.
The VCO 111, which has its oscillation frequency varied in accordance with the phase error detection signals, feeds back to the A/D converter 106 channel clocks having such a frequency that the phase error of the channel clocks with respect to the servo pattern reproduction data is equal to zero.
As can be seen from the foregoing description, the section of the magneto-optical disc driving apparatus which forms the channel clock has the structure of a so-called phase-locked loop (PLL) which outputs channel clocks whose phase is synchronized with the phase of the servo pattern reproduction data.
When the channel clocks of the phase synchronized with the phase of the servo pattern reproduction data are ready to be outputted, that is, when the phase locking of the servo pattern reproduction data is terminated, the magneto-optical disc driving apparatus terminates the servo pattern detection mode to enter a reproduction mode for reproducing recording data recorded on the magneto-optical disc 101.
In the reproduction mode, the magneto-optical disc driving apparatus reads out the recording data recorded on the magneto-optical disc 101, using the optical pickup 102. As described above, the reproduction signals from the optical pickup 102 are supplied via the amplifier 105 to the A/D converter 108, and are supplied also to a demodulating circuit 103.
The A/D converter 108 forms the reproduction data based on the channel clocks, as described above, and supplies the reproduction data to the latch circuit 107. This allows the servo patterns to be latched to form the phase error detection data. On the basis of the phase error detection data, variable control of the oscillation frequency of the VCO 111 is conducted repeatedly.
The demodulating circuit 103 demodulates the reproduction signals from the amplifier 105 based on the channel clocks, and supplies the demodulated reproduction signals to a speaker, not shown, via an output terminal 104.
Since the channel clocks have the phase synchronized with that of the servo pattern detection data, as described above, the reproduction signals can be correctly sampled and demodulated by the demodulating circuit 103 to output correct reproduction signals.
If the laser beam with which the magneto-optical disc 101 is irradiated is correctly on the track, the laser beam moves along a locus A shown in FIG. 4a. Thus, the substantially upper half of the first wobble pit, the entire clock pit and the substantially lower half of the next wobble pit are irradiated with the laser beam. For this reason, if the laser beam is correctly on the track, the servo pattern has a reproduction waveform such that waveform portions of the wobble pits lower in level than a waveform portion of the clock pit are on both sides of the waveform portion of the clock pit, as shown in FIG. 4b.
However, if the laser beam is not correctly on the track due to an external disturbance to the magneto-optical disc driving apparatus or due to a seek operation in which the optical pickup 102 is moved in the radial direction of the track to jump the track for access, the entire first wobble pit, the entire clock pit and the entire next wobble pit are irradiated with the laser beam, as shown by a locus B in FIG. 4a, or the entire clock pit is irradiated with the laser beam with the first and next wobble pits being scarcely irradiated with the laser beam, as shown by a locus C in FIG. 4a.
As the laser beam is not correctly on the track, as described above, the servo pattern may have a reproduction waveform such that the waveform portions of the clock pit and the wobble pits are substantially at the same level, as shown in FIG. 4d. The servo pattern may also have a reproduction waveform such that the waveform portions of the wobble pits are at an unusually low level with the waveform portion of the clock pit being at the normal level, as shown in FIG. 4c.
The phase error data for driving the VCO 111 is calculated by subtracting from the data the levels of both shoulders (a2, a1 and c2, c1) of the waveform portions of the wobble pits, adding the subtraction data, and dividing the addition data by 2, as shown by the Equation 1. Therefore, if the servo pattern has the unusual reproduction waveform due to the external disturbance or the seek operation, the addition data divided by 2 which normally indicates a constant value, that is, the phase error data, is changed to an unusual value. As the VCO 111 is driven by the phase error data of the unusual value, the PLL loop gain is fluctuated to cause an error in the channel clock phase. As a result, the track address cannot be reproduced correctly.